I may change my opinion as my child gets older, but I don't think it's a prerequisite to know the language. In fact, part of what they continue to learn at home is English, reading, writing, and how to convey ideas. I ask my child every night what the instructions are for her homework and we use a Spanish dictionary together. I could be wrong, but if a child does not readily grasp concepts, I think immersion might be an added layer of complexity that might be hard for a student.